This invention pertains generally to electronic circuitry for use with solid state electro-optical sensors of electromagnetic radiation, such as infrared (IR) sensors, and more particularly pertains to an output circuit usable with a focal plane array (FPA) of IR sensors, such as a scanned IR-FPA and a staring IR-FPA, both cooled and uncooled.
In modern electro-optical sensor systems, solid-state electronic devices are used to perform the function of sensing incident radiation at the image plane of the system, integrating this signal, multiplexing the integrated signal, and then driving the signal to the system""s electronics. Examples of such solid-state electronic devices are the visible CMOS and charge-coupled devices (CCDs) used in video cameras, as well as the IR-FPAs that are used in many civilian and military systems.
With most modern electro-optical sensors it is desirable to increase the output signal bandwidth and the data rate while reducing the power dissipated in the output circuitry. This is in part due to the increasing size, pixel count, and frame-rate that are characteristic of modern electro-optical sensors.
In so-called second generation IR-FPAs, two principal component materials are used to fabricate the devices. These are the incident radiation sensing detector material and the readout integrated circuit material. The detector material is chosen and optimized for sensing specific incident radiation wavelengths of interest, while the readout integrated circuit material is selected for its properties in realizing the desired signal processing and multiplexing functions.
FIG. 1 illustrates a conventional scanning IR-FPA 1. The two principal components are shown, i.e., the infrared detector array 2 and the scanning readout multiplexer 3. Incident radiation on the material of the detector array 2 generates a response in the detector material. This response is sensed, integrated, signal processed, multiplexed, and buffered by the scanning readout multiplexer 3. Suitable materials for fabricating the detector 2 may be Group III-V materials, such as InSb, or Group II-VI materials, such as HgCdTe, or other material types. Silicon-based technology is generally used to support the design and fabrication of the readout multiplexer 3. In the case of the scanning readout multiplexer 3, the image to be sensed is scanned across the surface of the detector array 2 and a line or series of lines is sensed at any given time. The image is therefore composed of a plurality of lines that are acquired over a period of time.
FIG. 2 illustrates a conventional staring-type of IR-FPA 1xe2x80x2. As in the case of the scanning IR-FPA 1 of FIG. 1, the infrared detector material 2 is mated to a readout multiplexer, in this case a staring-type readout multiplexer 3xe2x80x2. The staring IR-FPA 1xe2x80x2 differs from the scanning IR-FPA 1 in that at least a portion of all of the lines of the image can be sensed at one time. Thus, it is not necessary for the image to be scanned across the array 2.
In currently available state-of-the-art voltage mode IR-FPA output circuits, parasitic output capacitance limits the ability of the output (voltage mode) amplifier to slew and settle the output signal level at high frequencies. As a result, an increase in current is required to slew the output capacitance in order to obtain higher output signal bandwidths. In a related manner, lower impedance output MOS drivers are required to settle the output capacitance. Additional limitations in voltage mode output amplifiers arise at high output frequencies. These additional limitations arise due to the fact that as the signal frequency reaches the bandwidth limit of the amplifier, the amplifier experiences a significant reduction in the rejection of undesirable signals coupled from the power supply. For some electro-optical sensor applications many output amplifiers are required to support a total required output data bandwidth. In these cases the reduced power supply rejection can create problems, as the amplifiers may cross-talk to one another through the power supplies. Such cross-talk results typically in an objectionable degradation of the image quality obtained from the IR-FPA.
It is a first object and advantage of this invention to provide an improved IR-FPA that overcomes the foregoing and other problems.
It is a further object and advantage of this invention to provide an electro-optical sensor that has at least one differential current mode output amplifier that improves the overall signal bandwidth, power dissipation and power supply rejection characteristics of the electro-optical sensor readout electronics.
The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
In one aspect the teachings of this invention provide a novel current mode output amplifier circuit that generates a differential current output signal for electro-optical sensor arrays. The novel differential current mode output amplifier and related circuitry disclosed herein can readily provide at least a two to four times speed advantage over conventional voltage mode output stages, while operating at the same or lower power levels.
The teachings of this invention thus pertain to a circuit topology that provides a differential current output signal for electro-optical sensors. The described circuitry provides improved performance in the areas of signal bandwidth, power dissipation and supply rejection. The differential current mode output approach is well suited for use in wide bandwidth, low power applications, and is particularly well suited for those applications where a plurality of output amplifiers are required on a single electro-optical sensor.
The teachings of this invention have application to all types of electro-optical sensor arrays, particularly for those arrays where a wide bandwidth, low power, and multiple output amplifiers are required, and provides an important enabling technology for large-format, high-speed, low-power electro-optical sensors, such as advanced IR-FPAs.
In a further aspect the teachings of this invention pertain to readout integrated circuits (ROICs) as well as to electro-optical sensor arrays that are constructed so as to include at least one current mode output amplifier circuit that generates a differential current output signal.
A differential current mode amplifier circuit in accordance with the teachings of this invention includes a first circuit path or leg having a first current source providing a current I1 coupled in series with a first transistor (m1) at a first circuit node (n1). The first transistor has a control terminal for coupling to an input signal potential (Vs). Vs is obtained, by any suitable means, from a unit cell of a radiation detector array, and is indicative of a magnitude of photon-induced charge. The photons may be thermal (IR) photons, or visible light photons, or photons in other portions of the spectral band, such as the ultraviolet (UV). The first circuit leg outputs a first output current (Is). A second circuit path or leg includes a second current source providing a current I2 coupled in series with a second transistor (m2) at a second circuit node (n2). The second transistor has a control terminal for coupling to an input reference potential (Vr). The second circuit leg outputs a second output current (Ir). A resistance (Rs) is coupled between the first circuit leg and the second circuit leg at the first circuit node and the second node. The current flow through Rs is proportional to a difference between Vs and Vr, and is thus indicative of a magnitude of Vs.
An IR-FPA in accordance with the teachings of this invention includes an array of IR responsive photodetectors and a readout integrated circuit (ROIC) that is coupled to the array. The ROIC includes at least one differential current mode amplifier output circuit having a first input for inputting a first potential Vs that represents, at a particular time, a signal output from one of the IR responsive photodetectors. A second input of the at least one differential current mode amplifier output circuit is for inputting a second potential Vr that represents a reference potential. The at least one differential current mode amplifier output circuit outputs first and second currents, wherein a difference between the first and second currents is indicative of a difference between Vs and Vr. Other circuitry, such as a differential analog to digital converter fed by transimpedance amplifiers, can be employed to convert the difference in currents to digital values representing the output signals from the photodetectors.